DESCRIPTION (adapted from the Abstract): The Principal Investigator proposes to combine basic, clinical, behavioral, and epidemiological science in a descriptive observational study of PEP for sexual or parenteral exposure to HIV. A total of 500 persons reporting sexual or parenteral exposure to HIV within the previous 72 hours will be recruited at three sites: San Francisco General Hospital, The City STD Clinic, and a storefront prevention services clinic serving a primarily homeless population. Individuals will be interviewed, provided HIV antibody and viral load counseling and testing, informed about PEP, and provided chemoprophylaxis if they choose it. PEP will be provided over a one-month period, and all individuals (regardless of whether or not they elect PEP) will be followed for one year. This study will allow the Investigator and his associates to determine the proportion and characteristics of individuals who present with high risk exposure to HIV, the circumstances of their exposures, the proportion electing PEP, the proportion presenting for PEP who have undetected HIV, the degree of compliance with the PEP regimen, and the toxicities experienced by individuals taking PEP. They will also describe the proportion who develop HIV infection. Breakthrough infections will be studied for drug resistance. They will ask participants to allow us to contact (or to contact themselves) the partners who presumably exposed them to HIV (the source persons). The presumed source person will be asked to volunteer for the study in order to determine the percentage and characteristics of individuals infected with HIV and the viral resistance patterns of HIV if they are infected. A major aim of this study is to describe the potential harm to the individual and to the community. Individuals in the study will be followed for one year to determine the proportion and characteristics of individuals who increase high risk behavior during and after PEP, who contract an STD (syphilis, gonorrhea, or chlamydia) in the year following PEP, and who request additional courses of PEP during the follow-up year. The researchers will use epidemic modeling to determine if PEP will have a beneficial or adverse effect on HIV transmission in the community; they will formulate and analyze mathematical models to answer the following questions: How many risk episodes must be prophylaxed at any given efficacy and compliance rate? How many people need to receive PEP in order to reduce significantly transmission rates? If risk behavior increases, what level of risk behavior will produce a perverse outcome and result in increased transmission rates.